1. Field of the Invention
The present invention relates to a wire saw and a method of slicing a cylindrical workpiece of a fragile material such as a semiconductor ingot of silicon, gallium arsenide, or the like, so as to obtain wafers or wafer-shaped plates.
2. Description of the Related Art
In a wire saw, a wire is spirally wound around a plurality of rollers such that turns of the wire extend between rollers in parallel at a predetermined pitch. A cylindrical workpiece of a semiconductor ingot or the like or a quadrangularly prismatic workpiece of a synthetic quartz ingot or the like is pressed against the thus-arranged wire. Simultaneously, the wire is moved in its lengthwise direction, while a machining solution containing abrasives (hereinafter referred to as slurry) is fed between the workpiece and the wire, thereby concurrently slicing the workpiece into many (for example, several hundred) wafers having a predetermined thickness.
However, in contrast with a quadrangularly prismatic workpiece of a synthetic quartz ingot or the like, when a cylindrical workpiece of a semiconductor ingot is sliced using the above-described wire saw, an obtained wafer has a problem that thickness is different among a portion where slicing started, a central portion, and a portion where slicing ended.
That is, as shown in FIG. 1A, the thickness of the portion of a wafer where slicing started or ended becomes relatively thin, while the thickness of the central portion of a wafer becomes relatively thick. That is, the thickness of each sliced wafer varies such that the wafer is convex on both sides thereof. This variation of thickness is observed in almost all wafers which are obtained from the same workpiece in a single slicing process. Also, this variation of thickness is peculiar to the slicing of a cylindrical workpiece through the use of a wire saw and tends to intensify as the diameter of a cylindrical workpiece increases. Therefore, this variation of thickness raises a significant problem in slicing of wafers from a silicon semiconductor single crystal, because these wafers are required to have a uniform thickness, and the degree of integration of devices has been increased, so that the diameter of each wafer has been increased to 200 mm, 300 mm, or even more.